The present application relates to processing image data, wherein an object from or shown in a first clip of image frames is composited with the frames of a second clip on a frame by frame basis.
Advances in the field of digital signal processing have allowed many new developments to be effected in video and film post production. Many sophisticated image modifications and special effects have become widely accepted as part of post production procedures as a result of the developments of such products as xe2x80x9cFLAMExe2x80x9d, xe2x80x9cFLINT RTxe2x80x9d and xe2x80x9cFIRExe2x80x9d,licensed by the present Assignee.
A xe2x80x9cclipxe2x80x9d of film or video consists of a plurality of image frames which are displayed at frame rate to create the illusion of a moving image. The process of removing an object from a first clip and adding it to a second clip, to create the illusion that the object was always present in the second clip, is generally referred to as compositing.
Many compositing procedures were developed using highly labour-intensive procedures. Consequently, their application has been limited and, recently, a demand has developed for reducing the amount of labour required in order to achieve these effects, so as to make them more widely available.
When objects are moving within a scene, it is often necessary for an added object to exhibit some movement so as to maintain the illusion. A known process for achieving this is to perform a tracking operation upon a selected position within the background image. The tracking information is then used to effect movement of an added object over time, on a frame by frame basis. This approach has many useful applications but problems occur if, for some reason, it is not possible to maintain a track. This occurs, for example, if a tracked location becomes obscured by another object or if the track location extends beyond the image frame.
A display control system is disclosed in U.S. Pat. No. 4,908,700 in which a moving image of an object is successively compounded with a color picture image from an external source, so as to obtain a compounded image in which the resulting image of the object appears to be moving with respect to the color picture image. A display control apparatus includes a register unit for color range selecting data and circuitry for selecting one of first color image data corresponding to signals which are supplied from an external device such as a television camera and second color image data read from video memory. The value of color image data selected by the selection circuit is compared with a value of the color range selecting data stored in the register unit and based on this comparison either the first color image or second color image data is selected. This allows a first moving image to be inserted within a moving scene but the inter-relation between the two image portions is determined exclusively by the automatic selection made by the system, which, particularly at high definitions, may not always be appropriate.
According to a first aspect of the present invention, there is provided a method of processing image data, wherein an object shown in a first clip of image frames is composited with frames of a second clip on a frame by frame basis, comprising steps of determining a track of positions over time which specify a relationship between a point on frames from said first clip in relation to corresponding frames in said second clip; overlaying said object transparently over corresponding frames of said second clip; modifying the positions of said track; and compositing said images on the basis of said modified track positions.
The invention provides an advantage in that it is possible to make manual modifications to a tracked object. While modifications are being made, the objects to be added is overlaid but the overlaying process renders the object transparent, such that the underlying background can also be viewed. In this way, it is possible to make manual modifications to the track such that, during the final compositing procedure, the added object is positioned as required and thereby maintain the illusion in the output composited clip.
Preferably, tracked positions are determined by making pixel comparisons between adjacent frames. Preferably, an object is overlaid transparently by adjusting blending values.
According to a second aspect of the present invention, there is provided image processing apparatus, configured to display an object from a first clip of image frames composited with frames of a second clip, comprising determining means configured to determine a track of positions over time which specify a relationship between a point on frames from said first clip in relation to corresponding frames in second clip; overlaying means configured to overlay said object transparently over corresponding frames of said second clip; modifying means configured to modify the positions of said track and compositing means configured to composite said images on the basis of said modified track positions.
Preferably, said modifying means includes means to allow modification by manual operation.